Problem: In chemistry class, the girl to boy ratio is $7$ to $2$. If there are $8$ boys, how many girls are there?
Explanation: A ratio of $7$ girls to $2$ boys means that there is a group of $7$ girls for every group of $2$ boys. If there are $8$ boys, then there are $4$ groups of $2$ boys each. Because we know the ratio is $7:2$ , there must be $4$ groups of $7$ girls each. There is a total of $28$ girls in chemistry class.